The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andr e & Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
As for other printing plates it is required that the printing plates obtained according to the DTR-process have a high printing endurance, good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). It is furthermore desirable, especially for low printing runs, that the number of copies that have to be disposed of because of poor ink acceptance in the printing areas and/or ink acceptance in the non-printing areas during start-up of the printing process is limited.
In the preparation of these plates the exposure and developing step of the imaging element is in most cases followed by a neutralization step. This step is very important for the number of copies that have to be disposed of because of poor ink acceptance in the printing areas during start-up of the printing process. When the pH of the neutralization liquid is to high (i.e. higher than 8) the resulting printing plate has a (very) bad ink acceptance. It is thus very important that the neutralization liquid is refreshed in time in order to keep the pH of the neutralization liquid low enough.
This is especially important for printing plates made in a camera platemaker. This is an integrated apparatus wherein the exposure and the processing of the photosensitive material is executed in one apparatus and control of the neutralization liquid will require the stop of the total process of plate making. Furthermore, in many cases plates made in such a camera platemaker are used for printing a few days after their preparation while still new plates are made in said camera platemaker. If then is discovered that the ink acceptance of a plate is bad due to an exhausted neutralization liquid, the plates made in the mean time will also have a bad ink acceptance, requiring for all these plates an extra treatment with a starter to get a printing plate with a good ink acceptance. This is an ecological uninteresting, time consuming and thus costly processus, that is even not possible for plates with great dimensions.
In JP Pi 06-301216 a method for detecting the degree of exhaustion of a treating solution for a lithographic plate by incorporating an acid/base indicator in the solution is described. This solution has however serious drawbacks in that visual inspection of said liquid requires stopping of the camera platemaker and thus loss of production capacity and is in many instances practically impossible due to discoloration of the neutralization solution and the presence of sludge therein.